The present invention relates to array antenna systems, and more particularly to phased array antenna systems operating in two or more frequency bands.
Advanced airborne radar systems require lightweight phased array antenna systems capable of operating in two or more frequency bands that are conformable to an aircraft fuselage. The array antennas used for the two or more frequency bands must physically overlap because of the limited area available on an aircraft.
There have been many attempts to develop dual or multiband antenna systems. For example, extremely broad bandwidth or frequency independent elements such as log-periodic dipoles and spirals have been developed for applications other than phased arrays. For application in phased arrays, however, the elements must be spaced approximately one-half wavelength apart at all operating bands. This spacing obviously cannot be maintained with frequency independent elements in an array with many elements.
Other attempts to develop dual-band arrays have centered on combining two sets of elements where each operates over a limited bandwidth. One example is an array of dipoles operating at a low frequency band mounted in front of a slotted waveguide array operating at a higher frequency band. The dipoles and waveguide slots are cross-polarized to minimize interaction, and the waveguide array surface acts as a ground plane for the dipole array. Another example is the combination of an array of waveguide-dipole dual-polarization elements operating at a high frequency band with an array of waveguide or waveguide-dipole elements operating at a low frequency band. Both of these examples, however, require considerable depth behind the aperture planes, and are poorly suited for arrays conforming to a curved aircraft fuselage.
In another example of a dual-band array, stripline-fed cavity backed-slots operating at a high frequency band are interspersed with stripline-fed crossed cavity-backed slots operating at a low frequency band. The dielectric-filled cavities are on different layers of a complex multilayer stripline feed circuit. Although this array is conformable to an aircraft fuselage, the use of teflon-fiberglass dielectric materials makes it heavy for aircraft applications, particularly at low frequencies.